<body>
    <script>
        var a;
    console.log(a)    //undefined
    a = 10;              
    console.log(a)    //10
    var b = a;
    console.log(b);   //10
    var c = a + b;    
    console.log(c);   //20
    var d = c > a;
    console.log(d);   //true

    // a = a + 1
    // a += 1;
    // a *= 2;
    // a /= 3;
    // a %= 3;

    console.log(a); //10
    </script>
</body>